pmTool, the process run by Activity Monitor to actually collect
stats, appears to leak memory. If I leave Activity Monitor
running for a good period of time, when I check up on it pmTool
is often using over 100 MB of Real Memory. …

A highly non-scientific tests shows it to be adding one new leak
every time the UI in Activity Monitor updates (default is every
2 seconds). So if you haven’t changed the update frequency then
pmTool will leak 3 KB of memory every 2 seconds.

That’s a lot.

I’m pretty sure this leak only happens on Intel-based Macs; on my PowerBook, I can leave Activity Monitor open for days at a time and pmTool only uses about 800 KB of real memory. But until Apple fixes this leak, it’s useful info for anyone using Activity Monitor on Intel-based Macs.